FastPass will help you create individual render channel passes the easy way. Once you have individual channel images you can use these in post-processing and composing packages such as Adobe After Affects™ or Adobe Photoshop to further refine and control the resulting picture.
Rendering out passes has been always a common practise in EIAS, however, depending on what kind of pass you want to generate, the process involves a bit more than just a few clicks to activate or deactivate channel flags. Take a reflection pass for example: When rendering a reflection pass, you may want the textures applied to the objects being reflected to appear in the actual calculated reflections. In order to achieve this, you have to keep the Diffuse flag (and Bump if appliable) active, but then these objects will be rendered along with the reflection pass, so you must choose all the objects and make them “Reflection Object Only”. This requires a lot of good planning also for other passes. While this may not be a problem for small scenes, it can be a long and time consuming task to achieve in large scenes. A similar thing might happen when rendering out other channels, such as the shadows…
How can the FastPass shader can help in all this? FastPass is a kind of “Channel Isolator”. It will output at your command ONLY the selected channel flag without the need of connecting or disconnecting other Object properties. Want only the reflection? Just enable it in the FastPass interface and the generated image will contain only the reflection, with all other flags calculated and showing up in the reflection.